<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>练习</title>
  <style>
    input {
      color: #999;
    }
  </style>
</head>

<body>
  <input type="text" value="手机">
  <script>
    //1.获取元素
    var text = document.querySelector('input');
    // 2.注册事件，获得焦点事件
    text.onfocus = function () {
      if (this.value === '手机') {
        this.value = '';
      }
      // 获得焦点需要把文本框里面的文字颜色变黑
      this.style.color = '#333';
    }
    // 3.注册事件，失去焦点事件
    text.onblur = function () {
      if (this.value === '') {
        text.value = '手机';
      }
      this.style.color = '#999';
    }
  </script>
</body>

</html>